On Fri, Jul 2, 2021 at 2:21 PM smitra <smi...@zonnet.nl> wrote: > On 02-07-2021 03:50, Bruce Kellett wrote: > > On Fri, Jul 2, 2021 at 4:02 AM smitra <smi...@zonnet.nl> wrote: > > > > >> > >> This definition only works when you replace the real physical world > >> by an approximation obtained by taking an appropriate infinite scaling > >> limit that allows decoherence to involve an infinite number of > >> degrees of freedom. > > > > This is not true. You can have decoherence with the involvement of > > only a very small number of environmental degrees of freedom. The > > buckyball experiments show precisely this -- it only takes the escape > > of one or two IR photons of an appropriate wavelength to cause > > complete decoherence and the destruction of interference. > > You are then considering the reduced density matrix by tracing out some > of the degrees of freedom, in this case the IR photons. That's an ad-hoc > way of defining the reduced system, not much better than interpreting > part of a superposition as a world. >
No, I am not tracing out anything. I am looking at whether an interference pattern is formed or not. I don't have to detect the IR photons in order for the interference to be destroyed. >> You can do this by e.g. letting hbar tend to zero. While we > >> as macroscopic observers are in some sense close to this limit, the > >> world we actually live in only has a finite number of physical > >> degrees of freedom in a finite volume. And locality implies that in a > finite > >> time after some experiment, only a finite volume can be physically > >> affected by the experiments, therefore the decoherence is in reality > >> nothing more than an entanglement with a finite number of > >> environmental degrees of freedom. > > > > The important point to notice is that decoherence always involves the > > escape of IR photons at the speed of light. These are never > > recoverable, so the laws of physics ensure that the decoherence is, in > > general, irreversible. You have to take extreme care in very > > controlled settings to have things reversible. And if they are > > reversible, there can be no permanent environmental record of the > > result of the experiment, so one could reasonably say that no > > measurement has been made. > > It's implausible that escaping IR photons should be relevant for the > question of what an observer is, what observations are etc. How is it implausible? It is the inevitable existence of the IR photons that ensures that the measurement process is irreversible. It is the formation of permanent (irreversible) records in the environment that determines the existence of a measurement. If no such records are made then no measurement has been made. How can it matter whether or not very far away all IR photons are captured > and > billions of years later the entire original state is restored? > Irreversibility means that this is not physically possible. The escaping photons can never be restored with the correct phases. >> The exact physical state of the system plus environment therefore > >> does not become a mixed state. The fact that one cannot demonstrate that > >> the state after measuring a superposition is still a superposition using > >> an interference experiment does not mean that it isn't a superposition. > >> The observer itself has become entangled with the measured system, which > >> is the real reason why the observer cannot even in principle detect the > >> superposition. > > > > No. The real reason is that decoherence, and the recording of a > > result, is irreversible. > > These things are not irreversible in principle, only FAPP. Says you. The laws of physics, principally the limitation of the speed of light, means that the state cannot be restored, even in principle. Decoherence > only involves a finite number of degrees of freedom and can therefore be > simulated by a large quantum computer. Observers implemented virtually > in a quantum computer can perform measurements, the system will > decohere, but the entire setup can then be such that the original state > gets restored. How can it matter for the measurements that much later > the original state gets restored? > The trouble with this standard quantum computer response is that if no permanent record of the result is made, no measurement has been performed. And permanent records are, in principle, irreversible. And in case of our real universe, how can it be relevant that time > evolution is really irreversible? What if the universe is closed and > will end up evolving back in time according to exact time reversal > invariance in 10^40 years. How can that be relevant for measuring a > spin here and now? > This is just a fairy story. The universe is, in fact, open. Even if it were closed, it would never evolve back in time. >> The practical obstacle that the massive entanglement > >> involves an astronomically large number of degrees of freedom is of > >> course also true, but this cannot be physically relevant. > > > > Of course the irreversibility, even without involving a large number > > of degrees of freedom, is physically relevant. Whereas, the presumed > > persistence of the superposition in the mythical "universal wave > > function" is, indeed, physically irrelevant. > > > >> So, if you measure the z-component of a spin polarized in the > >> x-direction and I'm not aware of the measurement result, then my > >> mind will not have been entangled with the measurement result (you can > >> also put me outside your light cone for argument's sake). > > > > That does not always work -- consider entanglement and Bell pairs. > > Locality is not always true. > > Locality in the sense needed here is always valid, it's not violated in > Bell-type experiments. In those experiments you have to create an > entangled pair using local interactions and then bring those some > distant away from each other. But then then what is demonstrated in such > experiments is that local hidden variables don't exist in general. So, > we can then make use of the fact that when you measure the spin, the > result is actually not determined if I'm not aware of what you found, > which implies the existence of multiple worlds > The violation of the Bell inequalities implies non-locality; it does not show the existence of multiple worlds. You cannot get a local explanation of the Bell violations in MWI. >> The spin entangled > >> with you and a large but finite number of degrees of freedom will > >> therefore be in a superposition. The fact that hidden variables > >> don't exist means that it cannot be the case that you have made a > definite > >> observation that I'm unaware of. > > > > Of course that can be the case. It is the formation of a permanent > > record in the environment that is relevant to the existence of the > > measurement, whether you are aware of the result or not. You can be > > entangled with the spin-up state without being aware of it. > > The permanent record will then also be in a superposition (involving all > the degrees of freedom, including escaping IR photons). Part of my body > can be entangled with that superposition, but the relevant brain parts > that implements my mind will not be entangled If it were entangled then > that would imply that at least in principle, I could know the > measurement result, because the information about it it would be present > in my brain. > > > >> But obviously if I ask what you've > >> measured I'll always get an answer that I can verify to be correct. > >> So, the only way out of this problem is to assume that these > >> suppositions after measurements exist as different worlds where > different > >> experimental outcomes have been found. > > > > That conclusion does not follow. > > It follows from the assumption that QM is a fundamental theory, so we > then don't invoke effective macroscopic physics that is only true FAPP, > like escaping IR photons. > The irreversibility of measurement when there is a definite recorded outcome is the fundamental theory. Whether or not there is still a superposition in the mythical "wave function of the universe" is physically irrelevant. Which means that such a thing could not affect the outcome of any measurement now, or in the future. Bruce -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLReqYg7ZCTBV5n9bwxhYsAKPkoT-sxPG8Qcad5pLCWKGw%40mail.gmail.com.
